1. Field of the Invention
The present invention relates generally to radio-frequency (RF) and/or microwave components, and particularly to RF and/or microwave coupled transmission line components.
2. Technical Background
Communication systems typically require a number of sub-systems and components to convert baseband signals into RF signals for subsequent transmission over a communication channel. Conversely, RF signals received via the communication channel must be converted into baseband signals for use by the user and/or subscriber. Examples of such systems are ubiquitous and include cell phones, cable television converters, satellite television converters, etc.
It is often the case wherein one stage of the communication system employs differential (i.e., balanced signals) signals and a subsequent stage unbalanced signals. A differential signal includes two signal paths, each being 180° out of phase with the other. An unbalanced line is simply implemented as a single signal path. For example, certain antennas are balanced structures that require a balanced feed. However, the system may be such that the signal source is an unbalanced RF transmitter. This situation may also present itself in the opposite direction as well. A push/pull amplifier, for example, may provide a balanced differential signal for subsequent use by an unbalanced antenna. As those of ordinary skill in the art will appreciate, a balun is typically used to couple a balanced signal source to an unbalanced load (e.g., an antenna) or vice-versa. The word “balun” is shorthand for a balanced-unbalanced network.
Baluns are typically implemented using several coupled transmission lines, i.e., directional couplers. Couplers are four-port passive devices that are commonly employed in radio-frequency (RF) and microwave circuits and systems. A coupler may be implemented by disposing two conductors in relative proximity to each other such that an RF signal propagating along a main conductor is coupled to a secondary conductor. The RF signal is directed into an input port connected to the main conductor and power is transmitted to an output port disposed at the distal end of the main conductor. An electromagnetic field is coupled to the secondary conductor and the coupled RF signal is directed into an output port disposed at an end of the secondary conductor. The output signals are, of course, 90° out of phase with each other. An isolation port is disposed at the other end of the secondary conductor. The term isolation port refers to the fact that, ideally, the RF signal is not available at this port. At the isolation port, the incident signal and the coupled signal are substantially out of phase with each other and cancel each other out.
Those of ordinary skill in the art will appreciate that balun performance, weight, form factor and volume are important issues for most implementations. One commonly known balun implementation is referred to as a Marchand balun. The Marchand balun includes a main half-wavelength transmission line coupled to two quarter-wavelength transmission lines. The unbalanced port is connected to the half-wavelength structure. The quarter-wavelength transmission lines provide the differential signal ports. Each differential signal port accommodates a signal that is equal in amplitude and opposite in phase to the other differential port. The Marchand balun is limited in that it supports wideband applications only when the unbalanced impedance is lower than the impedance of the balanced ports. Typical impedance transformation ratios are 1:2 or 1:4. A variation of the Marchand balun is known as the Merrill balun.
The Merrill balun may be thought of as an inverted Marchand balun because the balanced signals are provided at either end of the half-wavelength structure. The unbalanced port is disposed at one end of one of the quarter wavelength transmission lines. The other quarter wavelength transmission line is grounded at both ends. The half-wavelength structure and the quarter wavelength elements may be implemented using stripline segments formed by disposing a layer of conductive material on a dielectric substrate. While the performance of the Merrill balun, as measured by insertion loss and return loss over a predetermined bandwidth, is adequate, there are drawbacks associated with this balun implementation. The Merrill balun is limited in that it supports wideband applications only when the balanced impedance is less than or equal to the unbalanced impedance. For example, typical impedance transformation ratios are 1:1 or 2:1. This is another reason why Merrill baluns are referred to in some quarters as inverted Marchand baluns. In many designs, the electrical length and the even-mode impedance are essentially fixed, only the odd-mode impedance may be manipulated to optimize performance. One drawback of both the Marchand and Merrill baluns relates to the excessive line-widths of the stripline structures at certain odd-mode impedance values.
In certain applications, system designers are requiring that the balanced ports of the balun are isolated from each other and ground. In each of the examples discussed above, there are direct current (DC) paths between the balanced ports and/or ground. As those of ordinary skill in the art will understand, DC isolation is typically implemented by coupling the differential ports of the balun to the balanced signal source/sink via decoupling capacitors. Thus, size reductions may be realized if decoupling capacitors could be eliminated from the design.
What is needed is a balun implementation having an isolated balanced port while conforming to a desired form factor for a desired performance specification.